1. Field of the Invention
The present invention relates generally to lithography. More particularly, the present invention relates to maskless lithography.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or glass substrate. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithography each can require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is accomplished by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.
Conventional lithographic systems and methods form images on a semiconductor wafer. The system typically has a lithographic chamber that is designed to contain an apparatus that performs the process of image formation on the semiconductor wafer. The chamber can be designed to have different gas mixtures and/or grades of vacuum depending on the wavelength of light being used. A reticle is positioned inside the chamber. A beam of light is passed from an illumination source (located outside the system) through an optical system, an image outline on the reticle, and a second optical system before interacting with a semiconductor wafer.
A plurality of reticles is required to fabricate a device on the substrate. These reticles are becoming increasingly costly and time consuming to manufacture due to the feature sizes and the exacting tolerances required for small feature sizes. Also, a reticle can only be used for a certain period of time before being worn out. Further costs are routinely incurred if a reticle is not within a certain tolerance or when the reticle is damaged. Thus, the manufacture of wafers using reticles is becoming increasingly, and possibly prohibitively, expensive.
In order to overcome these drawbacks, maskless (e.g., direct write, digital, etc.) lithography systems have been developed. The maskless system replaces a reticle with a spatial light modulator (SLM) (e.g., a digital micro mirror device (DMD), a liquid crystal display (LCD), or the like). The SLM includes an array of active areas (e.g., mirrors or transmissive areas) that are either ON or OFF to form a desired pattern. A predetermined and previously stored algorithm based on a desired exposure pattern is used to turn ON and OFF the active areas.
FIG. 1 shows a conventional SLM-based writing system 100 using a flat SLM 102 as a pattern generator. Light from illumination system 104 is directed to SLM 102 via a beam splitter 106 and an optical system (not shown) that contains at least an optical element 108. After reflecting from SLM 102, light is passed through beam splitter 106 and directed to a substrate 110 via an optical system (not shown) having at least an optical element 112. In order to maintain a double telecentric beam towards SLM 102 and substrate 110, optical element 108 must have a same diameter as SLM 102. Beams can be considered double telecentric when a chief ray of each beam is parallel to an optical axis of the SLM 102 and/or parallel to an optical axis of the substrate 110. There are manufacturing limits as to how large a diameter optical element 108 can have (e.g., 300–350 mm). This, in turn, limits a size of SLM 102. Throughput is based on the size of SLM 102. Thus, by restricting the size of SLM 102 because of optical element 108, throughput is much lower than what could be obtained if a diameter of SLM 102 was increased.
Therefore, what is needed is a maskless lithography system having an SLM with any sized diameter, which is also not limited by a size of an optical element that may be used to project light onto and off of the SLM.